Voyager's Neptune flyby - threading a needle in the cosmos. Vast distances, few guide points test NASA's navigational team

Voyager 2 will not reach the planet Neptune until August 1989. But NASA controllers are already fine-tuning its flyby. The spacecraft is scheduled to come closer to this planet than it has ever approached any body. Parts of Neptune's rings lie across the original path.

That path, coming within 800 miles of the planet's cloud tops, also passes through its outer atmosphere. Frictional drag could slow the craft and cause it to burn up.

To avoid these hazards, planners at the National Aeronautics and Space Administration's Jet Propulsion Laboratory (JPL) say they probably won't let Voyager come closer than 3,000 miles from the cloud tops, still the closest planetary flyby.

Voyager project manager Richard P. Laeser says the Neptune encounter is one of the most demanding navigational challenges his team has yet faced.

The spacecraft will be skirting a large planet (15,400 miles in radius) and its major atmosphere-bearing moon Triton.

The Voyager project began in the late 1970s with two craft, Voyagers 1 and 2, and was originally intended to study only Jupiter and Saturn.

Voyager 1 did just that and is now heading out of the solar system with no more planets in its path.

Voyager 2, however, after its flybys of Jupiter and Saturn, was to go on to visit two more planets, Uranus, which it passed last January, and Neptune, in 1989.

By then the craft will have spent 12 years in space heading ever farther away from Earth. Its equipment has seen hard service and is now obsolete, but Mr. Laeser is confident that his team will continue to get the most out of it.

The navigators' ability to predict the positions of distant Neptune and its moons, however, is even poorer than it was during the flyby of Uranus early this year, Laeser says.

What makes the navigation efforts even more demanding is the fact that it will take several days to make a change in Voyager's instructions as it nears the planet.

Neptune, near the edge of the solar system, will be nearly 2.8 billion miles from Earth during the encounter almost two years from now.

For that reason, Laeser says, his team wants to do as much planning and navigational programming as it can in advance.

JPL is keeping the size of the project staff level rather than reducing it to caretaking status and then building it up for a crash planning effort as the spacecraft nears Neptune.

``We figure we're not going to learn much more about Neptune between now and [encounter],'' Laeser explains. ``That's why programmers can design the navigational sequences ... this early.''

The flyby now being considered would bring Voyager past Neptune around midnight Eastern daylight time on Aug. 25, 1989.

It is to sweep by Triton four hours later, passing within 24,000 miles of that moon, instead of 5,000 miles as originally planned.

The trajectory change, which the Voyager Science Steering Committee tentatively approved Nov. 17, won't be made until March 13. Its details may change in the meantime.

Nevertheless, it represents the kind of navigational fixes the Voyager team wants to make as early as possible, to minimize the last-minute course corrections at the planet.

Laeser expects there will be enough ultrafine navigational tuning to challenge his team, even with the best advanced course adjustments it can make.

He explains that the background star fields that Voyager uses for orientation will be ``kind of crummy'' this time.

Navigators will have only two moons to sight on, as far as is known now. Unless new moons are found, navigation will be very tricky. At previous planetary encounters, they had arrays of satellites to work with.

The spacecraft needs this guidance to point its instruments accurately as it zips by Neptune and Triton. Laeser says, ``We're going to be doing [guidance updates] in spades,'' as Voyager closes in on its target.